The present invention relates to a signal tracing control system for a programmable machine controller PMC, and more particularly to a signal tracing control system for a PMC which is capable of storing data in a tracing buffer having a small capacity.
When trouble occurs during software development or operation of a controlled device, it is necessary to know the stored contents or data items of various memories in order to investigate the cause of the trouble. In this manner, it is more important to understand dynamic conditions, i.e., the process of a change, than to know static conditions. One system for knowing dynamic conditions has been to alter the program of a control apparatus and monitor a certain signal. However, if the program of the control apparatus is stored in a ROM, it cannot be altered. A system for storing the contents of a memory for a certain period of time is known as a system for monitoring the contents of the memory.
If the contents of a memory are to be stored as they are, the capacity of a memory used to store them must be large. If a memory of a certain storage capacity is used, the time during which the memory contents are monitored is limited. Therefore, memory data responsible for the trouble may not be discovered.